Many types of foldable tailgate ramp systems have been used in the past. In most designs, the objective was to increase a deployed length and strength of the ramp without unduly increasing the size of its structural members. This objective remains a primary requirement to any acceptable foldable ramp concept.
For reference purposes, the foldable tailgate ramps of the prior art are believed to belong to three broad groups. The first group utilizes longitudinal beams for stiffening a ramp in use; the second group uses trusses or mid-span legs to strengthen the hinges between each section of the ramp, and the third group uses the abutting edges of adjoining panels for preventing the hinges from folding when the ramp is extended.
Examples of tailgate ramps of the first group wherein longitudinal beams are used to strengthen a ramp, are disclosed in the following patent documents:
U.S. Pat. No. 5,540,474 issued on Jul. 30, 1996 to Richard T. Holland,
U.S. Pat. No. 5,752,800 issued on May 19, 1998 to Darryl Brincks et al.
In the first example, there is disclosed a tailgate ramp made in two sections that are hinged to one another. The ramp sections are constructed with marginal structural channel members and the panels are held in a planar alignment with each other by flat bars extending inside the channel members of both sections. In the second example, four ramp sections are maintained in a planar alignment with each other along an inclined path, by three support beams extending under the ramp, along the full length of the ramp.
In the second group of tailgate ramps of the prior art, the hinges are reinforced by inverted collapsible trusses mounted thereunder, or by foldable leg members extending from a mid-span portion of the ramp to the ground. Some examples of these designs are found in the following documents:
U.S. Pat. No. 3,976,209 issued on Aug. 24, 1976 to Edward E. Burton;
U.S. Pat. No. 4,571,144 issued on Feb. 18, 1986 to Donald C. Guidry et al.;
U.S. Pat. No. 4,795,304 issued on Jan. 3, 1989 to James D. Dudley;
U.S. Pat. No. 4,944,546 issued on Jul. 31, 1990 to William K. Keller;
U.S. Pat. No. 5,312,148 issued on May 17, 1994 to Alfred W. Morgan;
U.S. Pat. No. 5,342,105 issued on Aug. 30, 1994 to Mark R. Miles;
U.S. Pat. No. 5,791,717 issued on Aug. 11, 1998 to Terry Gene Reich et al.
In a third group of tailgate ramp systems, the rotation of the hinges between the panels and an associated sagging of the ramp under load are restricted by the abutment of the edges of adjoining panels, or by stopper blocks acting upon the hinges. In this third group, the following examples are found.
U.S. Pat. No. 3,642,156 issued on Feb. 15, 1972 to Raymond A. Stenson;
U.S. Pat. No. 4,735,454 issued on Apr. 5, 1988 to John N. Bernard;
U.S. Pat. No. 5,133,584 issued on Jul. 28, 1992 to Dennis M. McCleary;
U.S. Pat. No. 5,156,432 issued on Oct. 20, 1992 to Dennis M. McCleary;
U.S. Pat. No. 5,273,335 issued on Dec. 28, 1993 to Philip Belnap et al.;
U.S. Pat. No. 5,287,579 issued on Feb. 22, 1994 to Orestes Estevez, Jr.
While it is generally known in the field of Mechanical Design that the stiffness of a structure is directly proportional to its moment of inertia, none of the above tailgate ramp systems has means for advantageously blocking the hinges between the panels of a tailgate ramp, for causing the ramp to arcuate upwardly when deployed, thereby increasing its moments of inertia. Although the tailgate ramps of the prior art deserve undeniable merits, there is no known prior art that suggests that a strategic blocking of the hinges between the panels of a foldable tailgate ramp can substantially increase the strength of this ramp.